1. Field of the Invention
The present invention relates to a communication apparatus for receiving a plurality of signals from a plurality of base stations respectively and selecting one of them, which is the most suitable base station, to thereby communicate with the selected one.
2. Description of the Related Art
There is a communication apparatus adopting a CDMA (Code Division Multiple Access) method. By this CDMA method, the transmission signal is narrow-band-modulated and is further spread-spectrum-modulated by using a spread code series, to be thereby transmitted as a communication signal having a wide band. By performing such a spread spectrum modulation, it is possible to improve the capability of eliminating a disturbance wave and an interference wave, as compared with an ordinary narrow band modulating method. Further, by performing the spread spectrum modulation using the different spread code series for each user, a large number of users can commonly use the same frequency band to communicate with the other party. Namely, a so-called multiple access is enabled. Because such an advantageous feature is appreciated, the CDMA method is remarked as a basic technique for the movable body communication system in the next generation.
By the way, in the field of the movable body communication system adopting the CDMA method which has been already engaged in the business e.g., in the field of the portable telephone, such a system structure is employed that a large number of base stations, which are wire-line-connected to a communication line controlling station, are constructed to be distributed for each area (service area) and that a wireless communication is performed between the communication apparatus (portable telephone), which each user possesses, and each base station. By automatically detecting the closest base station to the portable telephone of each user from among the large number of base stations and then by communicating with the detected base station, it is possible to establish a good communication even if each user is moving.
In order to automatically detect the closest base station, the portable telephone may have a receiving circuit as shown in FIG. 5, for example.
In FIG. 5, in order to establish an asynchronous communication between the portable telephone and a certain base station among a large number of base stations, respective one of the base stations is identified by a downstream scramble code. The total: number of scramble codes usable here is 512. Those 512 scramble codes are divided into 32 groups, and the number of the scramble codes in each group is 16 (since 512 [codes] is divided by 32 [groups], 512÷32=16 [codes]). By using this group division, a cell search is performed.
Each base station spread-spectrum-modulates and transmits the assigned scramble code group identification short code and the scramble code. This transmitted electric wave is received by the portable telephone, so that it is possible to detect from which base station the received electric wave is.
In the receiving circuit show in FIG. 5, an electric wave from a base station is received by an antenna 1, and a reception signal thereof is amplified by an RF (Radio Frequency) circuit 2 and is supplied to multipliers 3 and 4. To the multiplier 3, a code series for inverse-spreading, which is generated by a spread series generator 6, is supplied. To the multiplier 4, a code series for inverse-spreading, a phase of which is shifted by xcfx80/2 by a phase shift circuit 5, is supplied.
An inverse-spreading circuit is constituted by the multipliers 3 and 4, the phase shift circuit 5 and the spread series generator 6. The in-phase component (I component) generated by the multiplier 3 and the orthogonal component (Q component) generated by the multiplier 4 are supplied through low pass filters 7 and 8 respectively to a correlation calculating circuit and a complex matched filter 11.
The complex matched filter 11 detects the I component and the Q component supplied from the low pass filters 7 and 8. A scramble code synchronous detecting circuit 12 synchronous-detects (or synchronous-captures) the scramble code on the basis of this wave component detection result. Further, a scramble code group ID detecting circuit 13 detects the scramble code group identification short code IDn of the base station, on the basis of this synchronous detection result.
Namely, as the scramble code synchronous detecting circuit 12 performs the synchronous-detection as mentioned above, one scramble code group identification short code IDn is generated from among 32 scramble code group identification short codes ID1 to ID32. The scramble code group ID detecting circuit 13 detects the group, to which the base station belongs, on the basis of this scramble code group identification short code IDn.
A scramble code generating circuit 14 sequentially generates 16 scramble codes CODE1 to CODE16, which belong to this detected scramble code group identification short code IDn, and supplies them to the correlation operating circuit 9.
The correlation operating circuit 9 obtains the correlation between (i) the scramble code CODEm and (ii) the I component and the Q component from the low pass filters 7 and 8 respectively, each time when the scramble code CODEm among the 16 scramble codes CODE1 to CODE16 is supplied from the scramble code generating circuit 14, and supplies the correlation value Rm obtained by this correlating operation, to a scramble judging circuit 15. Through the correlation operating circuit 9, the I component and the Q component are supplied to a phase correcting circuit 10, so that the decoding process is performed by the phase correcting circuit 10 and finally a reception output is generated.
The scramble code judging circuit 15 judges that the identification of the base station (e.g., the synchronous capture) is completed if the correlation value Rm reaches or exceeds a predetermined threshold value THD while comparing the correlation value Rm with the threshold value THD. Then, the data communication or voice communication by using the pertinent receiving circuit is started. On the other hand, the scramble code judging circuit 15 judges that an appropriate base station is not detected if the correlation value Rm does not reach the threshold value THD. Then, the scramble code synchronous detecting circuit 12 performs the synchronous detection again, and the processes of synchronous-capturing as described above are repeatedly performed until the base station is identified.
However, in the above described communication apparatus, by detecting the scramble code group identification short code IDn from, the received electric wave, the groups are narrowed down to one group, to which the base station transmits this electric wave belongs. Further, by selecting one scramble code CODEm belonging to this one group, the closest base station is identified.
Accordingly, it takes a long time period to search the closest base station. Namely, in addition to the requirement of selecting the scramble code group, it is necessary to repeat the above mentioned various processes, 16 times at the maximum in order to select one scramble code CODEm from among the 16 scramble codes CODE1 to CODE16, even after the group identification short code IDn is detected.
It is therefore an object of the present invention to provide a communication apparatus and a communication method, which can speedily specify an appropriate base station from among a plurality of base stations.
The above object of the present invention can be achieved by a communication apparatus for communicating with one base station among a plurality of base stations. The communication apparatus is provided with: a present position detecting device for detecting a present position of the communication apparatus and outputting present position information indicative of the detected present position; an identification information obtaining device for obtaining identification information of respective one of the base stations on the basis of the present position information outputted by the present position detecting device; and a specifying device for specifying said one base station on the basis of the identification information obtained by the identification information obtaining device.
According to the communication apparatus of the present invention, the present position of the communication apparatus is detected by the present position detecting device, and the present position information is outputted. Then, the identification information of respective one of the base stations is obtained by the identification information obtaining device, on the basis of the present position information outputted by the present position detecting device. Finally, said one base station (i.e., the base station with which an appropriate receiving condition can be obtained) is specified by the specifying device, on the basis of the identification information obtained by the identification information obtaining device.
For example, one or a plurality of identification information of one or a plurality of base stations located close to the present position are obtained by the identification information obtaining device, on the basis of the present position information generated in advance by the present position detecting device. Then, an actual communication is tentatively performed on the basis of the identification information. At this time, if an appropriate receiving condition is obtained, the base station corresponding to the identification information when the appropriate receiving condition is obtained is specified as the appropriate base station by the specifying device.
In this manner, since the present position is detected in advance, it is possible to specify the base station, with which an appropriate communication can be established, for a relatively short time period.
In one aspect of the communication apparatus of the present invention, the identification information obtaining device includes a memory device for storing a plurality of position information of the base stations and the identification information in correlation with each other, to thereby obtain the identification information corresponding to the present position information from the identification information stored in the memory device.
According to this aspect, the identification information corresponding to the present position information is obtained from the identification information stored in the memory device, in which a plurality of position information of the base stations and the identification information are stored in correlation with each other. Thus, it is possible to specify the base station for a relatively short time period.
In this aspect, the memory device may further store category information to categorize the plurality of position information of the base stations respectively into a plurality of groups. The identification information obtaining device may obtain the category information of said one base station on the basis of the present position information, and may obtain the identification information on the basis of the obtained category information.
By constituting in this manner, by the identification information obtaining device, the category information of said one base station is firstly obtained on the basis of the present position information, and then, the identification information is obtained on the basis of the obtained category information. Thus, it is possible to specify the base station for a relatively short time period.
In another aspect of the communication apparatus of the present invention, the identification information obtaining device obtains one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position on the basis of the present position information. The specifying device specifies said one base station by selecting one identification information from among one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position.
According to this aspect, one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position (e.g., the closest base station, the second closest base station, the third closest base station and so on) are obtained on the basis of the present position information. Then, one identification information is selected from among one or a plurality of identification information corresponding to those base stations, to thereby specify said one base station. Thus, it is possible to specify the base station for a relatively short time period.
The above object of the present invention can be also achieved by a communication method of communicating with one base station among a plurality of base stations. The communication method is provided with: a present position detecting process of detecting a present position of the communication apparatus and outputting present position information indicative of the detected present position; an identification information obtaining process of obtaining identification information of respective one of the base stations on the basis of the present position information outputted by the present position detecting process; and a specifying process of specifying said one base station on the basis of the identification information obtained by the identification information obtaining process.
According to the communication method of the present invention, the present position of the communication apparatus is detected by the present position detecting process, and the present position information is outputted. Then, the identification information of respective one of the base stations is obtained by the identification information obtaining process, on the basis of the present position information outputted by the present position detecting process. Finally, said one base station (i.e., the base station with which an appropriate receiving condition can be obtained) is specified by the specifying process, on the basis of the identification information obtained by the identification information obtaining process.
In this manner, since the present position is detected in advance, it is possible to specify the base station, with which an appropriate communication can be established, for a relatively short time period.
In one aspect of the communication method of the present invention, the identification information obtaining process includes a storing process of storing a plurality of position information of the base stations and the identification information in correlation with each other into a memory device, to thereby obtain the identification information corresponding to the present position information from the identification information stored in the memory device.
According to this aspect, the identification information corresponding to the present position information is obtained from the identification information stored in the memory device, in which a plurality of position information of the base stations and the identification information are stored in correlation with each other. Thus, it is possible to specify the base station for a relatively short time period.
In this aspect, the storing process may further store into the memory device category information to categorize the plurality of position information of the base stations respectively into a plurality of groups. The identification information obtaining process may obtain the category information of said one base station on the basis of the present position information, and may obtain the identification information on the basis of the obtained category information.
By constituting in this manner, by the identification information obtaining process, the category information of said one base station is firstly obtained on the basis of the present position information, and then, the identification information is obtained on the basis of the obtained category information. Thus, it is possible to specify the base station for a relatively short time period.
In another aspect of the communication method of the present invention, the identification information obtaining process obtains one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position on the basis of the present position information. The specifying process specifies said one base station by selecting one identification information from among one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position.
According to this aspect, one or a plurality of identification information corresponding to one or a plurality of base stations close to the present position are obtained on the basis of the present position information. Then, one identification information is selected from among one or a plurality of identification information corresponding to those base stations, to thereby specify said one base station. Thus, it is possible to specify the base station for a relatively short time period.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.